The Issue with Diffusive Shock Acceleration

TL;DR

This paper examines recent advances in diffusive shock acceleration theory, revealing steeper particle spectra than standard predictions and proposing a new theory that aligns better with observational data from astrophysical sources.

Contribution

It introduces a novel theory of DSA that accounts for steep spectra due to strong magnetic turbulence, improving agreement with observations.

Findings

Simulations show steeper spectra than the traditional E^{-2} prediction.

Standard assumptions of DSA are violated in turbulent conditions.

Proposed theory better matches observational data from supernova remnants and cosmic rays.

Abstract

We discuss the recent developments in the theory of diffusive shock acceleration (DSA) by using both first-principle kinetic plasma simulations and analytical theory based on the solution of the convection/diffusion equation. In particular, we show how simulations reveal that the spectra of accelerated particles are significantly steeper than the $E^{-2}$ predicted by the standard theory of DSA for strong shocks, in agreement with several observational pieces of evidence. We single out which standard assumptions of test-particle and non-linear DSA are violated in the presence of strong (self-generated) magnetic turbulence and put forward a novel theory in better agreement with the particle spectra inferred with multi-wavelength observations of young SN remnants, radio-supernovae, and Galactic cosmic rays.